In recent years, there has been a drastic increase in the networking of control units, sensors, and actuators by a communication system, i.e. by a bus system, in the manufacturing of modem motor vehicles and machine construction, in particular in both the machine tool sector and automation. Synergistic effects may be achieved here due to the distribution of functions among a plurality of control units. These are known as distributed systems. Communication between different stations is increasingly being accomplished via at least one bus or at least one bus system. Communication traffic on the bus system and the access and reception mechanisms, as well as error handling, are regulated by a protocol.
The CAN (controller area network) protocol is well established in the automotive sector. This is an event-driven protocol, i.e. protocol activities such as sending a message are initiated by events which have their origin outside the communications system. Unique access to the communication system or bus system, is triggered via priority-based bit arbitration. A prerequisite for this is that a priority is assigned to each message. The CAN protocol is very flexible. It is, thus, readily possible to add additional nodes and messages as long as there are still free priorities (message identifiers). The collection of all messages to be sent in the network, together with their priorities and transmitter nodes, plus possible reception nodes, are stored in a list known as the communication matrix.
An alternative approach to event-driven, spontaneous communication is the purely time-triggered approach. All communication activities on the bus are strictly periodic. Protocol activities such as sending a message are triggered only by the advance of a time valid for the entire bus system. Access to the medium is based on the apportionment of time periods during which a transmitter has an exclusive transmission right. The protocol is comparatively inflexible; new nodes may only be added if the corresponding time ranges have already been freed up in advance. This circumstance requires that the order of messages already be determined before starting operation. Thus, a timetable is drawn up which has to meet the requirements of the message with respect to rate of repetition, redundancy, deadlines, etc. One speaks of a so-called bus schedule. The positioning of messages within the transmission periods must be coordinated with the applications which produce the message content to minimize the latency between the application and the transmission time. If this matching does not take place, the advantage of the time-controlled transmission (minimal latent jitter when sending the message on the bus) is destroyed. Thus, high demands are made on the planning tools. TTP/C is such a bus system.
The requirements outlined above for a time-triggered communication and the requirements for a certain measure of flexibility are met by the method of time-triggered CAN known as TTCAN (time-triggered controller area network), which is described in German Published Patent Application No. 100 00 302, German Published Patent Application No. 100 00 303, German Published Patent Application No. 100 00 304 and German Published Patent Application No. 100 00 305, as well as in ISO Standard 11898-4 (currently in the form of a draft). TTCAN meets these requirements by establishing the communication cycle (basic cycle) in so-called exclusive time windows for periodic messages of certain communication users, and in so-called arbitrating time windows for spontaneous messages of a plurality of communication users. TTCAN is essentially based on a time-triggered, periodic communication, which is clocked by a user or node, which gives the operating time and is known as the time master, with the help of a time reference message, or, for short, reference message. The period until the next reference message is known as the basic cycle and is subdivided into a predefinable number of time windows. In this context, a distinction is made between the local times, i.e. local timers of the individual users, i.e. nodes, and the time of the time master as the global time of its timer. Additional principles and definitions based on TTCAN are given in the ISO Draft 11898-4 or the mentioned related art and are thus assumed to be known and will not be explicitly described again.
Thus, there are numerous real time bus systems for networking control units in automation, in motor vehicles or elsewhere, including the aforementioned CAN, TTP/C or Byteflight, as well as the TTCAN just mentioned. CAN, TTCAN and Byteflight are single-channel bus systems, which means that redundancy may be achieved by duplicating the corresponding system. TTP/C is intrinsically a two-channel system, i.e. the redundancy is always built in. Many bus systems offer a time base synchronized with the bus as a service. In the bus systems designed from the start as two-channel or multichannel solutions, the synchronization per design is typically forced in that a node or user must simultaneously transmit on the two buses. This has advantages (e.g. synchronization is always ensured), but it also has a number of disadvantages, such as the fact that not every bus may be operated by itself, the time patterns on the two buses may only differ to a very limited extent, and the modularity of the two or more bus systems is weakened due to the coupling which is provided by design.
As explained, it is obvious that the related art is incapable of yielding optimum results in every respect. This situation is to be improved in the following.
In the case of buses or bus systems designed as single-channel systems, synchronization is performed explicitly if needed. In the following, a TTCAN network is assumed to be a bus system, or it is assumed that there are a plurality of TTCAN buses or bus systems, and that they are coupled, but this is only to be understood as restrictive with regard to the object of the present invention to be explained later, inasmuch as the properties of the TTCAN are a prerequisite or necessity for representing the object according to the present invention.